Small-beam calibration by 0.6- and 0.2-cm3 ionization chambers

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Abstract
Small beams are often applied in radiotherapy [of human neoplasms], e.g., in shrinking field and high-dose techniques with curative intent. For a given beam, measured field size factors (FSF) may vary with responses of different detectors to nonuniform radiation fluence. Dose rates of small photon and electron beams with diverse profiles were measured in polystyrene with 0.6- and 0.2-cm3 Farmer ionization chambers. FSF of 60Co, 4-, 8- and 16-MV photon beans, and of 6-, 12- and 20-MeV electron beams, were determined as ratios of dose rates of beams with differing field dimensions to dose rates of 10 .times. 10 cm beams. 60Co and 4-MV photon beam FSF were also determined in air using acrylic buildup caps. FSF obtained via 0.6- and 0.2-cm3 chambers are compared as functions of beam flatness and quality. It is shown that notable discrepancies can exist between FSF for the same field obtained with different detectors, even when chamber volumes are well within nominal beam dimensions. Possible dose delivery errors arising from use of the 0.6-cm3 chamber were from 0-2% for 4 .times. 4 to 5 .times. 5 cm photon beams, and from -1% to 5% for electron beams 4 cm in diameter. Possible errors > 5% were noted for most beams < 3.5 .times. 3.5 cm. Consequently, it is recommended that detectors smaller than the Farmer 0.6-cm3 chamber be employed in determining FSF of beams .ltoreq. 5 .times. 5 cm. For facilities where smaller detectors are unavailable, beam profile averaging and extrapolation of rectangular FSF are examined as corrective techniques for calibration of small beams by a relatively large detector.

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